handling checked checkboxes PHP

Answer 1

foreach($_REQUEST['mark'] as $value){
    echo "$value was selected\n 
";
}

if you want to know which one wasn't selected then store all possible selections into an array and walk ofer this array and do someting like

foreach($poss_select as $key=>$val){
    if(!in_array($val,$_REQUEST['mark']){
        $not_selected[$key] = $value;
    }else{
        deleteRow($value);
    }
}